1. Field of the Invention
The present invention relates to vehicle navigation systems, and more particularly to road map correction feedback for tightly coupled combinations of global position system (GPS) receivers and dead-reckoning in vehicles.
2. Description of Related Art
GPS navigation systems with street map displays are now widely available in most new vehicles throughout the world. New map updates are purchased and installed with DVD disks, and seem to come out every year on an annual basis. The typical GPS navigation receiver used in these vehicles is accurate to tens of meters in good environments but can degrade to even a hundred meters in dense urban environments. These vehicle navigation systems assume the vehicle is traveling on the mapped roads, and therefore use the map information to snap the vehicle position icon to the exact path of the nearest road. But if GPS reception is lost, as commonly occurs in tunnels and parking garages, the user position on the map display screen will simply not update.
Factory-installed navigation systems are generally expected to provide higher reliability and accuracy compared to after-market portable navigation devices. Drivers will not accept long start-up times or navigation failures that result simply from driving into tunnels or parking garages. Conventional factory-installed navigation systems have therefore been connected to the speedometers, odometers, inertial sensors like gyros and accelerometers and vehicle data buses to access speed, forward/reverse, and distance traveled information. However, the inertial sensors impose a higher cost.
Getting any heading information in conventional systems has required that expensive and delicate, directly coupled automotive gyroscopes be installed. SiRF Technology (San Jose, Calif.) markets a solution they say eliminates this relatively expensive sensor, and instead uses sensor data already available from other vehicle subsystems. According to SiRF, the sensor-data measurements must be accurate enough to extract meaningful information for dead-reckoning, the sensor measurements must be provided with sufficient frequency, and any delays in sensor-data delivery to the dead-reckoning system must be short, e.g., under ten milliseconds latency.
Most new automobiles in North America and Europe are equipped with vehicle speed sensors, and anti-lock braking systems (ABS) as standard equipment. Compasses are more frequently found in the USA than in Europe, but stability control is more popular in Europe than in the USA. Such dead-reckoning software has to take into account the different availability of particular sensors. The earlier SiRFdrive 1.0 was configured for vehicle platforms with a directly coupled gyroscopes, and the later SiRFdrive 2.0 used distributed ABS-module sensor data instead. SiRFdrive 2.0 computes individual wheel speeds to determine vehicle 110 speed as well as its rotational velocity, so the expensive gyroscope could therefore be eliminated. The SiRFdrive 2.0 dead-reckoning system is described in press releases as collecting the wheel turning ticks, or speed pulses, from each wheel. It further determines the calibration values that it will need for precise dead-reckoning. The better the resolution of the wheel-tick data, e.g., the larger number of wheel pulses per revolution, the better will be the overall dead-reckoning performance.
Since wheel turning tick data are read directly from a vehicle bus, there was seen no need to connect to the wheel sensors themselves. However, using raw ABS data is not so easy. Dead-reckoning requires the highest resolution at low speeds, since the largest heading changes usually occur at lows speeds while rounding turns at road intersections. The main function of an ABS module is to control lockups of the individual wheels that occur when hard braking system is applied at high speed. So, some ABS modules do not send measurements at the full resolution they are capable of, and the wheel sensor ABS modules may output inaccurate or no data at all at the lowest speeds. In these instances, the ABS software can usually be upgraded to provide the necessary resolution and low-speed data outputs needed by dead-reckoning applications. Advertizing literature says the SiRFdrive 2.0 has advanced algorithms to minimize the adverse effect such data loss has on accuracy at low vehicle speeds.
Unfortunately, such wheel-tick driven dead-reckoning systems have not lived up to their promise. What is still needed is a practical system that combines GPS and dead-reckoning and that includes self-calibration and correction algorithms that really work.